Modern steam turbines for electric power generation require rotor shaft forgings of various sizes and mechanical properties. For high pressure (HP) turbines and intermediate pressure (IP) turbines, in particular, those of large size and intended to be used at high temperatures, 12% Cr--Mo--V steel, 12% Cr--Mo--V--Cb--N steel and 12% Cr--Mo--V--Ta--N steel have been used since they provide a good combination of high strength, toughness at high ambient temperatures, and high creep rupture strength, which are prerequisites for such components as rotor forgings, for HP, IP or HIP turbines in fossil-fuel fired power plants.
In recent years, new applications have been proposed which require HP and IP turbines to operate at higher temperatures than present state-of-the art turbines. The aforementioned steels containing 12% chromium, however, do not have sufficient creep and creep rupture strengths for such new designs. Therefore, it has been desired to develop a new heat resistant steel which is superior in creep rupture strength at much higher temperatures.